12 March 2010
'SPONGE EFFECT' LIMITS IMPACT OF RAIN
By far the most common question we’re asked is why storages haven’t gone up much after Melbourne receives heavy rain.
During the recent storms, Melbourne’s catchments had 65mm of rain, yet storages went up by just 0.2%. A similar rain event last year in October – one of our wettest months – saw storages rise by 0.8% (see table below).
| RAIN EVENT | Rainfall | Run-off | Storage increase | Amount of rain in previous 30 days |
|---|---|---|---|---|
| 6-10 March 2010 | 65mm | 4.6 billion litres | 2.7 billion litres or 0.2% | 86mm |
| 13-17 October 2009 | 59mm | 17.4 billion litres | 15 billion litres or 0.8% | 162mm |
It’s all got to do with the conditions in the 160,000 hectares of forested catchments that generate most of Melbourne’s drinking water, particularly the soil moisture.
Catchment soils can be 10-15 metres deep, so if you think of them as a sponge, in summer and autumn the ‘sponge’ is typically dry and absorbs a lot of the rain, leaving little to run off into the dams.
This effect has been made even more pronounced by 1997-2009 being the driest 13 years on record for Melbourne’s catchments.
Compounding this is the fact that summer rain tends to be sporadic, so a one-off rain event will only serve to make the soils moist before drying out again. Also, the ground dries much faster in the heat of summer than during colder months.
Winter and spring rain events are typically more consistent, and once the soils are wet enough, more of the following rain will make its way into the dams.
So a rain event of 60mm in summer or autumn will generally be far less effective in boosting storages because much of the water will be soaked up by dry soil.
The one consistent trend is that the biggest storage gains are made when there are consecutive days and weeks of rain, rather than one-off storms. Typically, this happens in winter and spring.
Media Contact: Nicolas McGay +613 9235 2278 +61 (0)438 981 836